Traditional fluid filter cartridges for fluids such as engine oil have been almost entirely of the disposable kind in which a pleated paper filter element is employed. Such filter cartridges are generally used in a full and partial flow modes, all or part of the engine oil being passed through the filter of the filter element prior to re-circulation through the engine bearings and other moving parts requiring lubrication. Inevitably, the filter becomes progressively occluded by debris from the engine and from the combustion process. Equally inevitably, the filter cartridge must be replaced periodically and to facilitate this, the well-known spin-on, spin-off construction is now widely used. The used filter cartridge is simply thrown away and replaced by a new one at each oil change. However, such disposal creates environmental problems due to the difficulty of removing oil from the spent filter cartridge. To minimize environmental problems either a reusable filter cartridge with a cleanable filter element, or a much longer period between oil changes, or both, would be desirable.
It has been proposed that oil changes should be at greater intervals, but this imposes considerably greater demands on the oil filter and in particular on its ability to hold debris without at the same time developing an unacceptably high back pressure or restriction to fluid flow. One way of addressing this problem is to use a centrifugal cleaner of the kind in which oil under pressure is admitted to a rotor from which it escapes through nozzles which cause the rotor to spin about its axis. The centrifuge thus formed serves to remove particulate matter very efficiently, it only being necessary to clean out or even replace the rotor after very high mileages, in the case of a vehicle, or after a very extended running period, in the case of a stationary engine.
However, as a practical matter, it is not desirable to run a centrifugal cleaner of this self-powered kind in a full flow environment. It is usual to pass about 10 percent of the total flow through the centrifugal cleaner and the rest of the flow through a filter, either of the conventional paper element kind or through at least a relatively simple wire strainer which can be dismantled for periodic cleaning.